The present invention relates to a flow sensor for measuring the flow velocity or flow rate of a fluid flowing in a channel and, more particularly, to a thermal flow sensor.
Two types of flow sensors are known as thermal flow sensors for measuring the flow rate or flow velocity of a fluid. According to the first type, the spatial temperature distribution of a fluid is localized by the flow of heat emitted from a heat-generating body (heater), and the nonuniformity is detected by a temperature sensor (indirect heated type). According to the second type, a change in power or in resistance occurring when the heat of a heat-generating body is deprived of by a fluid is detected, thus detecting a flow velocity or flow rate (self-heating type).
Conventionally, a flow sensor of this type is mainly used for a noncorrosive gas, and recently, one that can be used for liquid or a corrosive gas as well has been developed. For example, a flow rate sensor disclosed in Japanese Patent Laid-Open No. 4-295724 (prior art 1) is known. Also, a thermistor flow velocity sensor and liquid flow rate sensor (prior art 2) disclosed in Japanese Patent Laid-Open No. 8-146026 are known.
In the flow rate sensor described in prior art 1, first, second, and third regions are formed on the first surface of a silicon substrate. A heat-generating body is formed on the first region, and a thermometer constituting portion is formed on the second region. The first and second regions are insulated and separated from each other by a porous silicon region obtained by oxidizing the third region. The second surface on a side opposite to the first surface serves as a surface that receives the flow of the fluid. A silicon cap is fixed to the first surface to increase the rigidity of the silicon substrate, and to protect the heat-generating body and thermometer constituting portion.
In the flow velocity/flow rate sensor described in prior art 2, a heat-generating body and its electrodes are formed on one surface of a plate-like substrate made of alumina, SiO2, or the like. The heat-generating body is covered with an insulator. A thermistor for measuring the temperature of the heat-generating body, and its electrodes are formed on the insulator. The other surface of the substrate is fixed to the inner surface of a cover (container) through an adhesive. Thus, the sensor is completely isolated from the fluid. The cover is made of a metal, e.g., stainless steel (SUS316L), which has good thermal conductivity and good corrosion resistance against a fluid to be measured. Hence, the reliability can be improved more than in the flow rate sensor of prior art 1 described above without causing any problems such as wear and corrosion.
In the flow rate sensor described in prior art 1, however, the silicon substrate is directly exposed to the fluid. Hence, this flow rate sensor cannot be used for a corrosive gas or liquid which is used in a semiconductor manufacturing apparatus and the like.
In the flow velocity/flow rate sensor described in prior art 2, the sensor is fixed to the inner surface of the cover through the adhesive. Accordingly, the heat conduction efficiency between the liquid and the sensor decreases, and the heat capacity of the sensor increases, so that the sensitivity and response speed decrease. Also, the characteristics vary depending on the use amount of the adhesive.
The present invention has been made to solve the conventional problems described above, and has as its object to provide a flow sensor which can cope with almost all corrosive fluids and in which the response characteristics and sensitivity can be improved.